Free radical polymerization is an important method for producing polymers from vinyl-containing monomers, conjugated diene monomers, or mixtures of either or both. Early on, free radical polymerization was accomplished via traditional radical chain processes, which provided very limited opportunities for creating polymers with rationally designed architectures. However, more recently, the discovery of techniques for inducing “living” characteristics to free-radical chain polymerization has revolutionized the field by providing methods for producing structurally well-defined polymers and copolymers. Collectively, these methods for so-called “controlled free radical polymerization” represent a powerful tool for producing polymeric structures, ranging from homopolymers with narrow molecular weight distributions to block copolymers to complex nanostructured polymer architectures. Known methods for controlled free radical polymerization include: atom transfer radical polymerization (ATRP), reversible atom fragmentation transfer (RAFT) polymerization, nitroxide-mediated polymerization (NMP), degenerative iodine transfer polymerization (DT), and diphenylethylene-mediated polymerization (DPE).
The methods for controlled free radical polymerization are based on the use of a control agent to introduce reaction pathways for reversible formation of dormant polymer chains from growing macroradicals. Under typical conditions, the equilibrium position of the reversible reaction is shifted strongly toward the dormant species, which lowers the concentration of macroradicals to the point where the rate of termination by bimolecular reactions (e.g., radical combination) is negligible compared to the rate of propagation. Each type of controlled free radical polymerization relies on a different type of control agent, and the utility of each process depends greatly on the nature of the chemical functionality created by the control agent's presence in the dormant polymer chains.
It is known to use sulfur-containing molecules, such as dithioesters, trithiocarbonates, xanthates, and derivatives of dithiocarbamates, as control agents for free radical polymerization, such as for RAFT polymerization. Many of these sulfur compounds are highly colored. To that end, the chromophores responsible for such coloring can become incorporated into the resulting polymer during free radical polymerization. This can lead to a noticeably colored polymer product, which is undesirable for many potential applications. In addition, the color-causing chromophores can participate in a number of chemical and photochemical reactions, including reactions that lead to cleavage of polymer chains.
Thus, it would be desirable to provide sulfur compounds for use as control agents for synthesizing polymers by free radical polymerization, particularly RAFT polymerization, that minimizes or overcomes the present drawbacks of current sulfur-containing control agents.